Environmental Engineering Reference
In-Depth Information
There has been decades of experience in the operation of fixed bed reactors, though for the
hydroprocessing of light feeds. Progressively, fixed bed reactors have been modified to achieve
the steady and prolonged operation using heavier feeds. The degree of modification increased
with the increasing amount of asphaltenes and metals in the feed. Therefore, it is believed that
for atmospheric distillates derived from conventional crudes, desirable conversions could be
achieved with a single fixed bed and/or a fixed bed comprising several layers of different
catalysts. Fixed bed reactors consisting of several sections in the same vessel may also be
suitable.
For metals- and asphaltenes-containing feeds, frequent shutdowns of the operation and catalyst
replacement could not be avoided using single fixed bed reactors. This problem can be
alleviated by using several fixed bed reactors connected in a series. In this case, the primary
function of the first reactor, termed as “guard reactors”, is to remove most of the metals with
the aim to extend catalyst life in the downstream reactors. A high HDM activity and/or metal
storage capacity is the requirement for the catalyst to be used in the guard reactor.
In some cases, a “guard chamber” is placed upstream of the guard reactor, which operates
mainly in the HDM mode. The function of the former is the removal of inorganic solids
dispersed in heavy feeds. Therefore, guard reactor is filled primarily with a catalyst possessing
a high metal storage capacity. At the same time, guard chamber is filled with the lower value
solid materials (e.g., clays, minerals, alumina, etc.) with the aim to filter off the inorganic
solids dispersed in heavy feed. Some removal of the V and Ni from heavy feed may be
achieved in the case that the guard material includes the
-Al
2
O
3
of a suitable porosity. Most
likely, part of these solids was formed during the non-catalytic reactions of V and Ni
porphyrins with H
2
and H
2
S rather than via catalytic reactions. The number of reactors
downstream of the guard reactor increases with increasing content of metals and asphaltenes in
the feed. Because of the different properties of the feed (product from the preceding reactor),
each reactor may require a different type of catalyst. Again, this depends on the origin of
the feed and anticipated slate of the products. Therefore, a special attention must be paid
during catalyst selection to achieve a synchronized operation of a multistage catalytic
system.
To avoid frequent shutdowns due to catalyst replacement, more advanced hydroprocessing
reactors, which have provision for either continuous or periodic addition and withdrawal of
catalyst during the operation, had to be developed.
Figure 3.10 [120]
shows that one type of
the advanced catalytic reactor employs an expanded and/or ebullated bed of catalyst, whereas
the other type employs moving beds. In the latter case, the catalyst is added at the top and
progressively moves towards the bottom for a periodic withdrawal co-currently with liquid
streams. In ebullated bed reactors, the slurry of catalyst in gas oil is continuously added at the
top and spent catalyst withdrawn at the bottom of the reactor. An ebullated bed reactor can be
operated without any difficulties even in the presence of inorganic solids dispersed in heavy
